The measurement of surface pressure and wall shear stress (skin friction) in laminar, transitional, and turbulent boundary layers requires that the sensing element be exposed to the flow. As such, it is necessary to minimize the surface roughness of the sensor to minimize disturbances to the surrounding flow field and provide increased measurement accuracy.
Electrical connection to conditioning electronics for microscale sensors typically occurs via wire bonds or bump bonding. Traditional wire bonds placed on the front side of pressure and wall shear stress sensors cause flow disturbances due to wire bond loop heights in excess of 100 μm, resulting in measurement errors and an increase in overall drag. Bump bonding can circumvent this flow disturbance by using electrical through-wafer vias (TWVs) that allow connection of the sensor to the conditioning electronics; however, the rigid connection of the sensor through the bump bonds can cause large packaging stresses that result in increased measurement error. Additionally, bump bonding and TWVs can lead to increased fabrication complexity and cost due to a larger number of processing steps and challenges associated with integration of the sensor fabrication process.
Despite years of effort, the ability to make continuous, real-time, direct measurements of wall shear stress with both mean and fluctuating components remains elusive. Because of this, investigations into fundamental fluid flow problems are often hindered. An instrumentation-grade tool to precisely measure wall shear stress would enable further research in the areas of skin friction drag and turbulent boundary layer analysis.